18:1 AFR is "cleaner"?

[Ecky]

Originally Posted by Gasoline Fumes

I see as high as 30-31 degrees in lean burn on my ScanGauge.

And if you drop out of lean burn with the same throttle position?

[Gasoline Fumes]

Originally Posted by Ecky

And if you drop out of lean burn with the same throttle position?

I'll check to be sure, but maybe 12 or so. Any more throttle and it quickly goes negative.

[RustyLugNut]

When you look at the graph on the first post, you notice the NOx spike just past the 16:1 AFR? That air to fuel ratio is your most economical mixture. It is the mixture that provides the most useful cylinder pressure for the least fuel burned. Why is that? It is because flame speed is still high and there is the additional "fuel" value of some of the nitrogen oxidizing! More power through pollution!

If nitrogen was not such a "lazy" reactant and required temperatures approaching 2300 deg C to oxidize, we would be able to use it as a fuel!

On the other hand, some of the builders of the first nuclear bomb had a fear that the fission explosion would set the atmosphere on fire. The nitrogen all oxidizing into NOx due to the temperature extremes caused by the nuclear fire. The physicists knew better and were not bothered, but lesser minds ran with the idea.

[serialk11r]

With California smog you're supposed to put all the stock parts back on, duh :P

At 18:1, you're losing around 20% of laminar flame velocity which sounds like a lot but it's not that many crank degrees. One idea is to look at the timing at a lower load value: if 50% load is -30 deg and 40% load is -35, I think -35 would be approximately in the ballpark. A lean mixture has higher thermal mass, but also a higher reaction rate due to more O2, and the two factors should cancel each other out in theory.

[RustyLugNut]

Originally Posted by serialk11r

At 18:1, you're losing around 20% of laminar flame velocity which sounds like a lot but it's not that many crank degrees. . . . . A lean mixture has higher thermal mass, but also a higher reaction rate due to more O2, and the two factors should cancel each other out in theory.

I agree that flame velocity is slower with increasingly lean mixtures but you then say it has higher thermal mass? And higher reaction rate? Remember, you have less fuel to provide the heat.

[serialk11r]

Originally Posted by RustyLugNut

I agree that flame velocity is slower with increasingly lean mixtures but you then say it has higher thermal mass? And higher reaction rate? Remember, you have less fuel to provide the heat.

Yes. This is comparing the same quantity of fuel, but more air (higher load, higher MAP).



There is excess air that absorbs heat. Same quantity of fuel => same quantity of heat, but more air. That decreases alpha.

There is excess oxygen. Reaction rate is proportional to concentration^order, you're increasing oxygen concentration while keeping fuel concentration the same, since the cylinder is still the same volume but you have higher partial pressure of oxygen. That increases omega (with the dot over it).

Going from 1.0 to 1.2 lambda isn't a massive change all things considered, so assuming the effects cancel out is approximately correct. You can always use less timing advance to be safe.

Side note: In the case of adding EGR, reaction rate is not affected much since the concentration of CO and NO is tiny, but you're also raising the temperature considerably which speeds up the reaction.

[RustyLugNut]

Originally Posted by serialk11r

Yes. This is comparing the same quantity of fuel, but more air (higher load, higher MAP).



There is excess air that absorbs heat. Same quantity of fuel => same quantity of heat, but more air. That decreases alpha.

There is excess oxygen. Reaction rate is proportional to concentration^order, you're increasing oxygen concentration while keeping fuel concentration the same, since the cylinder is still the same volume but you have higher partial pressure of oxygen. That increases omega (with the dot over it).

Going from 1.0 to 1.2 lambda isn't a massive change all things considered, so assuming the effects cancel out is approximately correct. You can always use less timing advance to be safe.

Side note: In the case of adding EGR, reaction rate is not affected much since the concentration of CO and NO is tiny, but you're also raising the temperature considerably which speeds up the reaction.

Concentration.

Omega naught is proportional to the concentration of reactants. In one instance, you are reducing one reactant ( gasoline ). On the other hand you are keeping the oxidizer concentration ( oxygen ) constant. Omega naught will be smaller.

Also, you are ignoring the other variable. Temperature. With less fuel and more air, T(sub b) is smaller ( the flame front burns at a lower temperature ). The temperature relation in the parentheses will be smaller.

At just this side of lean lambda, it is known that burn rate increases. But it rapidly falls off. I worked with three Chrysler vehicles and their lean burn systems. A four, six and V8. At 18:1 AFR, all of them needed quite a bit of ignition advance (60 DCA before TDC, average). However, you are correct in that the addition of EGR increased flame speed and ignition advance could be pulled back to 35-45 BTDC.